1. Field of the Invention
The present invention relates to a process for preparing a membrane comprised of an aromatic sulfone resin which has excellent heat and solvent resistance and is used for a selective permeability. The membrane of the present invention can be used for the separation of substances, for example, as a separating membrane and as a diaphragm for batteries.
2. Description of the Prior Art
The method of separating a substance through the use of a porous membrane having a selective permeability is generally called "membrane separation" and is widely utilized in various fields including desalination of sea water, production of pure water used in the electronics industry, processes for the food industry and treatment of industrial waste water. With diversification of the use of these membranes, a permselective membrane usable at a higher temperature in a wider pH range and capable of withstanding various kinds of chemical substances has become desired in the art. A porous membrane utilizing an aromatic polysulfone polymer as the membrane material and having excellent chemical resistance has been proposed in Japanese Patent Laid-Open No. 16381/1979. This porous membrane has been used as a permselective membrane or as a base membrane of a composite membrane. The above-described aromatic polysulfone polymer is characterized as being soluble in an aprotic polar organic solvent, etc. Thus a porous membrane could easily be prepared by dissolving said polymer in an organic solvent and forming a film from the resultant solution. Porous membranes may be made by a process called "phase conversion." Phrase conversion comprises processing a polymer solution by casting or the like and immersing the processed solution in a nonsolvent of said polymer, which is compatible with the solvent of said polymer solution, and allowing said solution to gel. By such a process it is possible to prepare a porous membrane having both a pore diameter suitable for the separation of the desired substance as well as asymmetric (anisotropic) membrane structure wherein the dense portion of the membrane surface (called an active layer where the separation is conducted) is supported by a structure called a porous layer comprising a network structure containing macrovoids, or the like which occupies a major portion of the membrane structure. The phrase conversion process is thus a very desirable process and has been used extensively for preparing membranes.
The aforedescribed asymmetric membrane structure has very excellent features. In particular, the dense layer, which is provided with minute pores having a separatory function, is very thin and has a very high permeability to a liquid to be separated. The membrane also has a very high mechanical strength since the thin dense layer is supported by the thicker porous layer. This porous layer generally has no separatory function. These features ensure easy handling of the membrane in practical use.
Further, the phase conversion method is used to form a porous membrane partly because the structure and pore diameter of the porous membrane can be adjusted according to the substance to be separated and the separation system by varying several parameters, such as the composition of the polymer solution, the composition of the immersion bath and the solvent to be evaporated [see "Maku ni yoru Bunriho (Method of Separation by Means of Membrane) edited by Bunji Hagiwara and Koichi Hashimoto, pp. 25-41, Kodansha Scientific (1974)].
For the production of porous membranes having enhanced characteristics, proposals have been made to use a membrane material comprising a polymer which barely dissolves or swells in an organic solvent; for example, engineering plastics having excellent heat and solvent resistances, such as a fluoropolymer described in Japanese Patent Publication No. 25332/1983, and a polyphenylene sulfide described in Japanese Patent Laid-Open Nos. 202659/1985 and 213813/1987. Since there is no solvent capable of completely dissolving these polymers, the porous material used to make these porous membranes must be prepared by a complicated method of melt molding at high temperatures, stretching after the molding, or extraction of additives; all requiring apparatus of a large size. Further, the structures and kinds of porous materials which can be produced are limited.
In the case of polyphenylene sulfone, although it is a polymer having excellent heat and chemical resistances, it can not be molded into a structurally useful porous membrane due to its high melting point and solvent resistance. Japanese Patent Publication No. 35370/1985 and Japanese Patent Laid-Open Nos. 213813/1987 and 225636/1988 propose the use of the above-described aromatic polyphenylene sulfone as a material for a porous membrane instead of an aromatic polysulfone polymer. An intended microporous molding, comprising an aromatic polyphenylene sulfone, is prepared by melt-molding the above-described polyphenylene sulfide, which is hardly soluble in an organic solvent and then chemically modifying the microporous molding. Therefore, although the above-described porous material per se has very excellent heat and chemical resistances, the structure and separatory function are substantially determined when molding the polyphenylene sulfide, which thus limits the structure and kind of aromatic polyphenylene sulfone which can be prepared. Therefore, as opposed to porous membranes prepared from the conventional polymer solution, this process produces membranes having only limited uses.
An asymmetric membrane has a structure wherein a very thin active layer is backed with a spongy support layer. After the benefits and advantages were of such a structure were recognized, numerous studies on the composite membrane were initiated. The objects of some of these studies was to prepare an asymmetric membrane by combining two types of polymers. The disadvantage of this such a process are that -two adjustment steps are required. The advantages are, however: (1) you can separately select materials suitable for an active layer and a support layer, (2) the active layer and the support layer can each be prepared by suitable methods, and (3) the thickness and porosity of the active layer can readily be regulated according to the application.
Studies on such composite membranes have become popular because the above-described aromatic polysulfone polymer was found to be an excellent support membrane material. Since polysulfone has excellent heat resistance, the active layer can be prepared on polysulfone by direction polymerization. At present, the common practice is to use the aromatic polysulfone polymer after reinforcement backing comprising a nonwoven fabric or the like is applied.
The aromatic polysulfone polymer used for preparing several composite membranes has been a driving force for the development of the composite membrane. However, excessive reliance on the support has inhibited the development of the composite membrane. Specifically, the aromatic polysulfone polymer is attacked by commonly used solvents, esters and ketones, so that the polymer material used for preparing the active layer is limited [see "Saikin no makuho tansuika gijutsu to atarashii datsuenyo maku (Recent Desalination Technique by Means of Membrane and New Desalination Membrane)", Chiyoshi Kamisawa, Maku (Membrane), 5 (6), 348-356 (1980)].
Japanese Patent Laid-Open No. 136107/1984 proposes a resin having a three dimensional structure as a membrane material having excellent chemical, solvent and heat resistances, mechanical strengths, etc. Specifically, this patent proposes a process for preparing a porous membrane having a three dimensional structure which comprises: (1) dissolving a resin partially containing a photosensitive group in a suitable solvent to prepare a solution, (2) forming a porous membrane from the solution, and (3) irradiating the porous membrane with light for crosslinking. This patent teaches that the porous membrane comprising a resin having a three-dimensional structure provides an ideal support for a composite membrane. However, it is difficult: (1) to complete the photoinduced crosslinking reaction, which renders the proposed method unsuitable for a continuous production of the porous membrane, and (2) to advance the crosslinking reaction into a depth of the membrane. Also irradiation with light from both sides of the membrane is necessary to insolubilize the membrane as a whole, thus rendering this method unsuitable for membranes which are tubular or hollow. Further, to effectively conduct the photo-induced crosslinking reaction, a sensitizer is usually used, which affects the function of the membrane. For example, the ultrafiltration performances (such as permeation flow rate and percentage solute cut-off) vary depending upon the kind and amount of sensitizer added. This membrane is also disadvantageous in that it slightly shrinks when subjected to the photo-induced crosslinking reaction and brings about a change (in many cases a lowering) in the membrane performance of, e.g., ultrafiltration. This membrane performance cannot be restored unless the membrane is treated by immersion in methanol or acetone.
Swiss Patent Nos. 491981 and 501028, U.K. Patent No. 1402314 and Japanese Patent Laid-Open No. 210130/1988 disclose polyaromatic sulfones.